An increasing demand for portable digital storage media for equipment such as digital cameras, handheld gaming consoles, and audio players has created a need for high-density nonvolatile memories. For such applications, flash memory technology has been successfully developed and employed.
As density and read/program bandwidth requirements continue to increase, flash memories are becoming inherently more complex, both from a design and manufacturing standpoint. A basic requirement for a flash process is a set of high voltage rules and associated masks that define the memory cell array, surrounded by its row and column decoding circuits. Triple well processes are commonly used and gate oxide thicknesses ranging from 180 Å to 300 Å are typically required for the flash cell and high voltage selectors. In addition to the matrix of cells and decoders, the memory control and the system interface logic must be integrated. This integration requires specific gate oxides, typically ranging from 40 Å to 90 Å, for low voltage CMOS devices.
With reference to FIG. 1, a typical monolithic memory device 100 includes a memory array 101 containing a plurality of memory cells (not shown), and an associated column decoder 103 and row decoder 105 for addressing specific locations of memory. One or more banks of charge pumps 109A, 109B provide a high voltage needed for programming of cells in the memory array 101. A PGM/ERASE/VFY data register 107 provides buffering capabilities for data to be stored in the memory array 101, and a memory control logic circuit 117 and a system interface logic circuit 115 allow for control of the memory device 100 and interfacing the memory device 100 with external circuitry and microcontrollers (not shown). External devices interface with the memory device 100 through a plurality of system pads 119a, 119b, . . . 119n. Additional analog circuits 111 and test circuits 113 are frequently included on contemporary memory devices 100 as well.
Components such as the analog circuits 111, the charge pumps 109A, 109B, voltage and current references, buffers, voltage comparators, and regulators (not shown), that are also commonly found as fundamental building blocks of flash memories, generally require specifically tuned components as well. For example, low threshold voltage, Vth, devices (or native devices), resistors, capacitors, and even integrated inductors frequently need to be specifically fabricated. Hence, as the lithography advances to produce smaller and consequently more cost effective devices, such integration requires a commensurate increase in the number of advanced masks and processing steps. This increased complexity results in increased manufacturing costs and limits an economic scalability of future memory devices.
Therefore, what is needed is a way to separate out the complex and dissimilar circuits from those which are more readily fabricated while minimizing the impact on speed, throughput, or reliability of the circuit device.